Its a bit of a work-around but the way that I get custom fields added to my subtasks is to add them to a project of their own as well as within the task. If that project has custom fields they will now appear in your subtasks. I then just use headers to separate subtasks out in their own sections (if you pre-build the section you can just selected from the dropdown next to the project within the task without going switching views) and now I also have a list view of all the subtasks for every task in my main project. It wouldn’t automatically add up the fields but it would make it easier to quickly grab those numbers to put in a calculator or export to an excel file for that purpose. You would add the subtask to a project by hitting Tab-P while in the edit view.
You might also want to check out the Harvest/Forecast integration - I don’t currently use it but Ive been looking into it and I think that it might fit your needs based on its functionality.
I hope that helps!